1. Field of the Invention
The present invention relates to a dehydration-sintering furnace, more particularly, a dehydration-sintering furnace for use in a dehydration and/or a sinter process of a porous optical fiber (glass fiber) preform.
2. Description of the Related Art
A porous optical fiber preform produced by Vapor Phase Axial Deposition (VAD) method, Outside Vapor Deposition (OVD) method and the like is dehydrated and/or sintered by accommodating the preform in a muffle of a dehydration-sintering furnace, supplying to the muffle dehydration agents such as chlorine, thionyl chloride or the like and an inert gas such as helium or the like, and then heating the muffle with a heater.
An example of a dehydration-sintering furnace for dehydrating and/or sintering the porous optical fiber preform is a furnace that has a muffle connected to a pressure fluctuation absorbing apparatus such as a balloon type pressure buffer, a gas-flow buffering chamber, or a pressure fluctuation absorption chamber utilizing an electro magneto valve and the like in order to suppress a rather large pressure fluctuation and swiftly keep the muffle pressure constant (Refer to, for example, Publication of Examined Utility Model Application H06-50513 and Japanese Patent Application Laid-open Publications H05-4828, H06-127964, and H10-120428).
While the pressure fluctuation absorbing apparatus connected to the muffle is advantageous in absorbing a short-term pressure fluctuation in the muffle, the absorbing apparatus is somewhat disadvantageous as described below.
When the pressure in the muffle is raised, the gas therein is pressed out into the pressure fluctuation absorbing apparatus from the muffle. On the other hand, when the pressure is lowered, the gas in the pressure fluctuation absorbing apparatus flows back to the muffle.
Since the pressure fluctuation absorbing apparatus is positioned outside of the furnace and exposed to a room temperature atmosphere, vapor that includes substances produced during a dehydration and/or sinter process is condensed (liquefied) in the pressure fluctuation absorbing apparatus. The larger amount of moisture exists in the porous optic fiber preform, the larger amount of condensation takes place.
The liquid produced by condensation (liquefaction) in the pressure fluctuation absorbing apparatus will flow back to the muffle when the pressure in the furnace decreases, thereby deteriorating the dehydration effectiveness in the muffle.
In addition, the liquid condensed in the pressure fluctuation absorbing apparatus may drop on the porous optical fiber preform and then taint the surface of the preform. Also, the dropped liquid may make brittle the portion of soot on which the liquid drops and then the portion may fall apart therefrom. Those will deteriorate the quality of the optical fiber preform and reduce the production yield of the same.
Also, when the liquid condensed in the pressure fluctuation absorbing apparatus does not flow back to the muffle but stays at a duct and the like connecting the muffle and the pressure fluctuation absorbing apparatus, since substances produced in the dehydration process contain strong acids such as hydrogen chloride and sulfuric acid, the duct and the like will be eroded, thereby shortening an operating life of the furnace.
The objective of the present invention is to provide a dehydration-sintering furnace having a pressure fluctuation absorbing apparatus, wherein vapor that contains substances produced in the dehydration and/or sinter process is prevented from condensing in the pressure fluctuation absorbing apparatus and thereby the flowing of condensed liquid back to the muffle of the dehydration-sintering furnace is essentially prevented.
Another objective of the present invention is to provide a manufacturing method of optical fiber preform, utilizing the above dehydration-sintering furnace, the method enabling a stable production of a high quality optical fiber preform.